1. Technical Field
The present invention relates to a powerless lens design capable of correcting for optical characteristics such as field curvature and spherical aberration.
2. Description of the Prior Art
In the design of any lens that will cover a finite field of view on a flat image surface, the most problematic aberration to correct, without sacrificing image quality or lens simplicity, is field curvature. Field curvature may be introduced into an optical imaging system as the incoming optical beam moves away from the optical axis of the system. So-called "field flattener" lenses have been developed over the years to address this problem. One such field flattener design is discussed in the reference Optics, Tenth Edition, M. H. Freeman, Butterworth (1990), in a section entitled "The achromatic doublet", beginning at page 467. The particular design includes a plano-concave field-flattening lens positioned near the image plane of the arrangement and an associated achromatic doublet. Other field-flattening arrangements use a large power lens (often negative) inserted between two lenses (positive) as, for example, in the Cooke triplet. The Cooke triplet has the disadvantage of requiring strong curvatures that introduce large aberrations. The aberrations can be made to somewhat cancel each other, but small amounts may remain.
Spherical aberration is a result of imperfect imaging with a spherical lens. This aberration may be present for both on-and off-axis illumination, since it is a result of deviation of peripheral (marginal) rays with respect to those rays located closer to the optical axis. A negative lens will exhibit negative spherical aberration; a positive lens will exhibit positive spherical aberration. The problem of spherical aberration is often addressed in the prior art by utilizing an achromatic doublet, providing both positive and negative spherical aberrations, such that the net effect is essentially zero.
A problem with any conventional lens arrangement for addressing these and other optical imaging problems is the amount of optical power the arrangement introduces into the imaging system, regardless of whether positive or negative corrective lenses are utilized. Further, the arrangements all require a number of separate optical components, which results in increasing the overall size, complexity and cost of the corrective arrangement. Thus, a need remains in the prior art for a simplified lens arrangement which corrects for characteristics such as field curvature and spherical aberration.